Bailey was a Theosophist who fell out with the Society's leadership and left in 1920 to found her own organisation, the Arcane School, with her husband Foster Bailey.
She said that from 1919 almost all of her books were dictated telepathically by a Tibetan adept she named Djwhal Khul, and that only a few of the twenty-four were her own. This is her account, and there is no way to test it; what can be observed is that the writing is continuous in style with the books she signed herself, and that the material develops Theosophical doctrine along lines Blavatsky had already laid down, the spiritual Hierarchy, the Masters, root races, the sevenfold constitution of the human being, with additions of her own, chiefly the seven rays as a typology of soul and personality.
Her influence on the later New Age movement is larger than her name recognition suggests. The phrase New Age itself, the Great Invocation, the idea of a coming world teacher, the Wesak festival observance, the vocabulary of meditation as world service, and much of the ambient language about the Aquarian age come through her writings and her school.
Two things in that legacy need stating plainly rather than being left to a footnote. Her cosmology retains the Theosophical scheme of root races and includes passages ranking human groups on a spiritual hierarchy, and her writings on Judaism contain sustained hostile generalisation about the Jewish people. Her defenders read these as period idiom or as referring to principles rather than peoples. That reading is available but it is strained, and readers coming to the books should know what is in them.